It is extremely desirable to develop thermoplastic elastomers of a rubber-like soft material, requiring no vulcanization, and having moldability like thermoplastic resins, for applications in the fields of automobile parts, household electric appliance parts, electric wire-protecting materials, medical appliance parts, miscellaneous goods, footwear, and the like. Various elastomer compositions containing the hydrogenated product of vinyl-substituted aromatic hydrocarbon/conjugated diene block copolymers (hereinafter referred to as hydrogenated block copolymer) have been used in thermoplastic elastomeric applications.
U.S. Pat. No. 5,710,206, to Francis et al, discloses gels containing block copolymers, polyphenylene ether and at least 500 parts of an extender oil per 100 parts by weight of the block copolymer. Such mixtures are too soft having low tensile strength and lower than desirable compression set.
WO 81/020020 discloses high-impact polyphenylene compositions comprising a polyphenylene ether resin, the hydrogenated block copolymer and an oil. The compositions obtained here provided thermoplastic resins having a good processability, but could not substantially provide thermoplastic elastomers superior in processability as well as compression set at 100.degree. C.
Japanese Pat. 89-49423 B teaches a composition of polyphenylene ether (PPO), hydrogenated styrene/butadiene/styrene block copolymer (SEBS) and a non-aromatic oil; wherein the parts by weight per hundred parts by weight of rubber hydrocarbon (hereinafter "PHR") of SEBS is 100 PHR; the weight proportion of SEBSIPPO ranges from about 90/1.about.30/70; the PHR of non-aromatic oil is from about 10.about.300; and, the composition has a compression set of less than 65% at 100.degree. C. Japanese Pat. 94-70162 B teaches a composition of PPO, SEBS and a non-aromatic oil; wherein the PHR of SEBS is about 100; the weight proportion of SEBS/PPO ranges from about 90/10.about.30/70; the weight proportion of non-aromatic oil to the sum of SEBS and PPO is greater than 0.43; and, the composition has a compression set of less than 65% at 100.degree. C.
It has long been recognized that two or more polymers may be blended together to form a wide variety of random or structured morphologies to obtain products that potentially offer desirable combinations of characteristics. However, in many cases, it may be difficult or even impossible in practice to achieve many potential combinations through simple blending because of some inherent and fundamental problem. Frequently, the two polymers are thermodynamically immiscible, which precludes generating a truly homogeneous product. This immiscibility may not be a problem per se since often it is desirable to have a two-phase structure. However, the situation at the interface between these two phases very often does lead to problems. The typical case is one of high interfacial tension and poor adhesion between the two phases. This interfacial tension contributes, along with high viscosities, to the inherent difficulty of imparting the desired degree of dispersion to random mixtures and to their subsequent lack of stability, giving rise to gross separation or stratification during later processing or use. Poor adhesion leads, in part, to the very weak and brittle mechanical behavior often observed in dispersed blends and may render some highly structured morphologies impossible.
The hydrogenated block copolymer-based thermoplastic elastomers produced according to the prior arts have a high-temperature (100.degree. C.) compression set of 65% or more, do not reach the required level of high-temperature compression set for vulcanized rubber applications. Consequently, hydrogenated block copolymer-based thermoplastic elastomer compositions that are molded repeatedly without losing their excellent high-temperature (100.degree. C.) compression set, often require the use of a fourth ingredient such as a polyolefin or polystyrene, or in other instances a curative such as a peroxide as shown in the prior art such as in U.S. Pat. No. 4,772,657, to Akiyama et al.
The use of low molecular weight oils, as required in the prior art to obtain soft gels, often results in an undesirable property, called bleeding; whereupon oil exudes to the surface of a molded part formed from such gels, resulting in potential contamination of the immediate area and increasing the hardness of the part. Furthermore, oils are readily extractable from a molded part containing oil when that part is bought into contact with cleaning fluids or aqueous solutions containing solvents or surfactants, thereby limiting the areas of use of such parts.
The present invention was made to solve the above problems that could not readily be solved with the conventional molding materials for elastomers. Particularly, it was found that thermoplastic elastomer compositions which can be processed easily and used repeatedly, and yet which are superior in high-temperature (100.degree. C.) compression set, can be obtained by a simple blending technique.